For example, in a shell-and-tube heat exchanger or the like, a plurality of heat-transfer pipes are carried in a shell such that a primary medium flowing through the pipes are heat-exchanged with a secondary medium flowing through the shell. It has been already proposed, for example, in undermentioned Patent Literatures 1 and 2 that outer peripheries of the heat-transfer pipes are formed with spiral grooves which provide spiral protrusions, as inverse formations, on inner peripheries of the pipes to swirl the primary medium flowing through the pipes, thereby increasing contact frequency and contact distance of the primary medium to the inner peripheries of the pipes to enhance the heat exchange efficiency.
Such structure with the plural heat-transfer pipes in parallel with each other in the shell has a problem that the heat exchanger as a whole becomes large in structure due to lowness in heat exchange duty per unit volume and thus has deteriorated mountability to instrument/equipment. Thus, as exemplarily shown in FIG. 1, it has been started to study a flat pipe a shaped like cylindrical pipes arranged planarly and mutually proximately and connected together at mutually proximate portions thereof as communicating portions, cylindrical pipe portions b of the pipe a corresponding to the above-mentioned cylindrical pipes having inner peripheries formed with swirling-flow-forming protrusions c along spiral trajectories coaxial with central axes O of the portions b so that swirling flows of the medium d may be individually formed in the respective portions b (Japanese Patent Application No. 2011-220778).